1. Field of the Invention
The present invention relates to apparatus for the measurement of changes in angular orientation between two plane mirrors. More particularly, the invention relates to optical apparatus which is useful for high accuracy angle metrology using interferometry.
2. The Prior Art
High accuracy displacement and angle measurements are required in the machine tool industry and in the semiconductor fabrication industry. Displacement is commonly measured with an interferometer. Angles are commonly measured with either an interferometer or an autocollimator.
While there are numerous interferometer configurations which can be used to measure changes in angular orientation between two plane mirrors, none provides a high speed electrical output which is insensitive to changes in the displacement between the two mirrors. In conventional interferometers, changes in angular orientation between two mirrors manifests itself, in general, as a change in fringe spacing and a rotation of the fringe pattern while changes in the displacement between the two mirrors manifests itself as a translation of the fringes. Thus, it takes rather complex, time consuming processing to separate these effects in order to extract the desired angular information. Therefore, for high accuracy angular measurement of precision, high speed X-Y stages used in microlithography, the prior art interferometers are not used.
An adaption of a displacement interferometer has been used to make angular measurements, see for example R. R. Baldwin, L. E. Truhe, and D. C. Woodruff, "Laser Optical Components for Machine Tool and Other Calibration," Hewlett-Packard Journal, pp. 14-16, April 1983. However, this apparatus measures the changes in angular orientation of a pair of retroreflectors, not plane mirrors. Thus, it measures changes in angular orientation of a part which can be displaced in only one dimension, i.e., displacements parallel to the direction of the incident laser beams. Therefore, this apparatus is also not useful for the high accuracy angular measurements of precision X-Y stages.
Autocollimators provide many of the desired characteristics, see for example D. Malacara, Optical Shop Testing, p. 467, John Wiley & Sons (1978). However, for high accuracy measurements, interferometers are preferred because their measurements are based directly on a stable, fixed, built-in measurement unit, i.e., the wavelength of light.
The present invention retains the preferred chacteristics of both the autocollimator and the interferometer while avoiding the serious limitations of prior art apparatus. In the present invention, the angular measurement is insensitive to not one but rather to three dimensional displacements of the plane mirrors, and the measurement is interferometric so that it is based on the wavelength of light. The improvements of the present invention thusly overcome the disadvantages of the prior art and allow the high accuracy, i.e., to a small fraction of an arc second, angular measurement required for precision, high speed X-Y stages.